The present invention relates to the field of magnetooptical recording systems and media. More particularly, the present invention relates to magnetooptical systems using magnetooptical disks on which data is recorded in both land portions and groove portions.
Generally, overwritable magnetooptical disks use a phase variable film or photomagnetic film as a recording layer. A laser beam is then focused to a spot on the magnetooptical disk and information is recorded and/or played back with the laser beam.
A recording method for this type of magnetooptical disk has achieved a higher density by recording information in both land and groove portions of the disk while shortening a wavelength of an incident laser beam. This type of recording method is generally referred to as a land and groove recording method.
In the land and groove recording method, recording density in a direction of track width is much higher compared to conventional magnetooptical disks which allow either land only, or groove only, recording.
Consequently, in the land and groove recording method, information that has been recorded in adjacent land or groove portions may inadvertently be erased when recording or erasing information. This is known as "cross erase."
One factor which contributes to cross erase is thermal conductivity. Accordingly, to reduce thermal conductivity between adjacent tracks, an optical disk may be provided with an increased difference in grade between land portions and groove portions. Thermal conductivity may also be reduced between adjacent tracks by creating a deeper groove in order to increase a thermal distance between adjacent grooves.
Nevertheless, conventional magnetooptical disks still have a problem in that an increased degree of complexity is required to control recording and playback power due to the increased difference in grade between the land portions and the groove portions.
FIGS. 7A (prior art) and 7B (prior art) are respective cross-sectional views of a conventional magnetooptical disk 98. FIG. 7A particularly illustrates recording of information in a land portion while FIG. 7B particularly illustrates recording of information in a groove portion. A deep grade difference is formed between land portion 90A (a convex portion) and groove portion 90B (a concave portion). As illustrated, reference numeral 91 represents a magnetic layer, reference numerals 95 and 96 represent protective layers, and reference numeral 97 represents a glass base.
Normally, each layer of the magnetooptical disk is formed through a film forming method such as sputtering, thereby resulting in a different film thickness in each layer of land portion 90A and groove portion 90B. This phenomenon is known as shadowing, wherein film thickness tb of recording layer 91B in groove portion 90B becomes less than film thickness ta of recording layer 91A on land portion 90A (e.g., approximately 60% of recording layer 91A) during film formation. In other words, groove portion 90B is in the shadow of land portion 90A.
As a result of shadowing, the recording and erasing powers are different in land portion 90A and groove portion 90B due to this difference in film thickness, i.e. ta&gt;tb.
For example, recording is executed with a lower recording power PB in groove portion 90B (having a thinner film thickness), whereas a higher recording power PA (PA&gt;PB) is required for recording in land portion 90A (having a greater film thickness).
In this case, at least two types of recording and erasing powers are required for communicating with each respective land portion and groove portion, thereby resulting in a more complicated configuration and more complex control for the device, as well as an associated increase in cost.
As illustrated in FIG. 7B, when erasing or recording information in groove portion 90B, recording layer 91A is not easily heated because film thickness ta of recording layer 91A is thicker than film thickness tb of recording layer 91B. Recording layer 91A has a greater thermal mass thereby reducing a potential for unintended erasure of information.
On the other hand, as illustrated in FIG. 7A (prior art), when erasing or recording information in land portion 90A, recording layer 91B is easily heated because film thickness tb of recording layer 91B is tin in comparison with film thickness ta of recording layer 91A. Recording layer 91B therefore has a smaller thermal mass which occasionally allows information in recording layer 91B to be erased due to heat from adjacent recording layer 91A. In this case, the use of a deep groove as a cross-erase prevention measure is ineffective.